(a) Except as provided in subsection (b) of this section, proper money of the claim is, as the case may be, the money: (1) Regularly used between the parties as a matter of usage or course of dealing; (2) used at the time of a transaction in international trade, by trade usage or common practice, for valuing or settling transactions in the particular commodity or service involved; or (3) in which the loss was ultimately felt or will be incurred by a party.
(b) The money in which the parties have contracted that a payment be made is the proper money of the claim for that payment.